5l6 SPECIAL PLANT PATHOLOGY 



plied. A germ tube is formed through which the antheridial con- 

 tents pass over into the oogonium. A single large central egg-cell, or 

 oosphere, becomes differentiated in the protoplasm of the oogonium; 

 this contains a single nucleus in a central position, while the remaining 

 nuclei pass into the peripheral layer of protoplasm (periplasm). A 

 single male nucleus passes through the antheridial beak into the 

 oosphere, which becomes surrounded by a cell wall. Nuclear fusion 

 now takes place and the oosphere becomes an oospore with a single 

 central nucleus. The oospores are about 30/x in diameter. 



Bordeaux mixture is the most important fungicide used in combating 

 the downy mildew disease. It is applied as in black-rot. 



